NOTE5

Acl2 now allows ``complex rationals,'' which are complex numbers
whose real parts are rationals and whose imaginary parts are
non-zero rationals. See complex.

A new way of handling forced hypotheses has been implemented.
Rather than cause a case split at the time the force occurs, we
complete the main proof and then embark on one or more ``forcing
rounds'' in which we try to prove the forced hypotheses.
See forcing-round. To allow us to compare the new handling of
force with the old, Version 1.5 implements both and uses a flag in
state to determine which method should be used. Do
(assign old-style-forcing t) if you want force to be handled
as it was in Version 1.4. However, we expect to eliminate the
old-style forcing eventually because we think the new style is more
effective. To see the difference between the two approaches to
forcing, try proving the associativity of append under both settings
of old-style-forcing. To get the new behavior invoke:

Events now print the Summary paragraph listing runes used, time,
etc., whether they succeed or fail. The format of the ``failure
banner'' has been changed but still has multiple asterisks in it.
Thm also prints a Summary, whether it succeeds or fails; but thm is
not an event.

The acceptable values for ld-skip-proofsp have changed. In the old
version (Version 1.4), a value of t meant that proofs and localevents are to be skipped. In Version 1.5, a value of t means proofs
(but not localevents) are to be skipped. A value of 'include-book
means proofs and localevents are to be skipped. There are two
other, more obscure, acceptable values. See ld-skip-proofsp.

Theory expressions now are allowed to use the free variable world
and prohibited from using the free variable state.
See theories, although it is essentially the same as before
except it mentions world instead of state. See world for a
discussion of the Acl2 logical world. Allowing in-theoryevents to
be state-sensitive violated an important invariant about how books
behaved.

Table keys and values now are allowed to use the free variable world
and prohibited from using the free variable state. See the note
above about theory expressions for some explanation.

The macro for minus, -, used to expand (- x 3) to (+ x -3) and now
expands it to (+ -3 x) instead. The old macro, if used in the
left-hand sides of rewrite rules, produced inapplicable rules
because the constant occurs in the second argument of the +, but
potential target terms generally had the constant in the first
argument position because of the effect of commutativity-of-+.

A new class of rule, :linear-alias rules, allows one to implement
the nqthm package and similar hacks in which a disabled function is
to be known equivalent to an arithmetic function.

A new class of rule, :built-in-clause rules, allows one to extend
the set of clauses proved silently by defun during measure and guard
processing. See built-in-clauses.

The rules for how loop-stoppers control permutative rewrite rules
have been changed. One effect of this change is that now when the
built-in commutativity rules for + are used, the terms a and (- a)
are permuted into adjacency. For example, (+ a b (- a)) is now
normalized by the commutativity rules to (+ a (- a) b); in Version
1.4, b was considered syntactically smaller than (- a) and so
(+ a b (- a)) is considered to be in normal form. Now it is
possible to arrange for unary functions be be considered
``invisible'' when they are used in certain contexts. By default,
unary-- is considered invisible when its application appears in
the argument list of binary-+. See loop-stopper and
see :DOC set-invisible-fns-table.

Extensive documentation has been provided on the topic of Acl2's
``term ordering.'' See term-order.

Calls of ld now default ld-error-action to :return rather than to
the current setting.

The command descriptor :x has been introduced and is synonymous with
:max, the most recently executed command. Historycommands such as
:pbt print a :x beside the most recent command, simply to indicate
that it is the most recent one.

The command descriptor :x-23 is synonymous with (:x -23). More
generally, every symbol in the keyword package whose first character
is #\x and whose remaining characters parse as a negative integer
is appropriately understood. This allows :pbt:x-10 where :pbt(:max -10) or :pbt(:here -10) were previously used. The old forms
are still legal.

The simplifier now reports the use of unspecified built-in type
information about the primitives with the phrase ``primitive type
reasoning.'' This phrase may sometimes occur in situations where
``propositional calculus'' was formerly credited with the proof.

The function pairlis has been replaced in the code by a new function
pairlis$, because Common Lisp does not adequately specify its
pairlis function.

The connected book directory, cbd, must be nonempty and begin and
end with a slash. It is set (and displayed) automatically upon your
first entry to lp. You may change the setting with set-cbd.
See cbd.

See ld-keyword-aliases for an example of how to change the exit
keyword from :q to something else.

The attempt to install a monitor on :rewrite rules stored as simple
abbreviations now causes an error because the application of
abbreviations is not tracked.

A new message is sometimes printed by the theorem prover, indicating
that a given simplification is ``specious'' because the subgoals it
produces include the input goal. In Version 1.4 this was detected
but not reported, causing behavior some users found bizarre.
See specious-simplification.

:Definition rules are no longer always required to specify the
:clique and :controller-alist fields; those fields can be defaulted
to system-determined values in many common instances.
See definition.

A warning is printed if a macro form with keyword arguments is given
duplicate keyword values. Execute (thm t :doc nil :doc "ignored")
and read the warning printed.

A new restriction has been placed on encapsulate. Non-local
recursive definitions inside the encapsulate may not use, in their
tests and recursive calls, the constrained functions introduced by
the encapsulate. See subversive-recursions. (Note added in
Version 2.3: Subversive recursions were first recognized by us here
in Version 1.5, but our code for recognizing them was faulty and the
bug was not fixed until Version 2.3.)

The defcor event, which was a shorthand for a defthm that
established a corollary of a named, previously proved event, has
been eliminated because its implementation relied on a technique we
have decided to ban from our code. If you want the effect of a
defcor in Version 1.5 you must submit the corresponding defthm with
a :by hint naming the previously proved event.

Error reporting has been improved for inappropriate in-theoryhints
and events, and for syntax errors in rule classes, and for
non-existent filename arguments to ld.

Technical Note: We now maintain the Third Invariant on type-alists,
as described in the Essay on the Invariants on Type-alists, and
Canonicality. This change will affect some proofs, for example, by
causing a to rewrite more quickly to c when (equiv a b) and
(equiv b c) are both known and c is the canonical
representative of the three.